Derek, I have to disagree with you on the separation/groundspeed thing. The aircraft and the jumpers aren't influenced by the ground or it's relative speed, the ground doesn't even exist for them (unless ya get too close). Aircraft and it's spawn, that's you and me, only know the air.

Airspeed for a particular jump ship tends to be constant for each jump run. If the jump ship always flies into the upper winds for jump run, then as the upper wind speed increases, the jump ship’s ground speed decreases and more time must be left between exits in order to achieve the same amount of separation. The pilot could increase the aircraft’s jump run speed equal to the upper winds (if the aircraft had the performance to do so) and the amount of time between exits could remain constant, but this isn’t done. Therefore, exit separation is a function of the aircraft’s ground speed on jump run. In order to ensure separation between groups the following groups must look at the ground and allow sufficient ground distance to be covered before exiting. This is why the 45-degree rule for exit separation does not work.

So, as a jump ship’s ground speed decreases, more time must be left between exits to ensure the same amount of separation. This is why I said that separation is a function of ground speed. Ground speed (for a particular jump ship) is the variable, since airspeed is constant. Ground speed is used to adjust the length of time between groups.

Hooknswoop, what you have just said roughly works for the normal case but.... Why do we keep having to redo the problem? What's even worse is there are a number people taking part in this thread that took part in all the other threads about exit seperation and they have not learned anything! It's obvious numerous people never even bothered to look at Kallend's web site, or if they did, they never understood what was presented there.

Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

Hooknswoop, what you have just said roughly works for the normal case but....

But what? It works for any case.

If you need 2,000 feet of separation from the group you are following out, do you wait for the aircraft to fly through the air for 2,000 feet or do you wait for the jump ship to fly 2,000 feet over the ground?

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Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

If you wait for the jump ship to fly through the air for 2,000 feet and it has zero ground speed, you will have zero separation, not 2,000 feet.

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That would work great in court.... Sorry judge I was not speeding, you see I was in reverse gear which means I was doing -40mph.

BTW If you do a jump run with the wind do you then have a negative ground speed?

If a Piper Cub is flying with a 40 knot airspeed into a 45 knot head wind, it has a –5 knot ground speed. If a jump ship is moving with the wind and not into the wind, relative to the ground, it has a negative ground speed.

What would you call it if a jump ship was backing up over the ground on jump run? “Going forward in reverse”? Sounds like something the French Army would say.

Hooknswoop, what you have just said roughly works for the normal case but....

But what? It works for any case.

If you need 2,000 feet of separation from the group you are following out, do you wait for the aircraft to fly through the air for 2,000 feet or do you wait for the jump ship to fly 2,000 feet over the ground? I do neither

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Ground speed is irrelevent for exit seperation, but in most cases it is a useable indicator to work out exit seperation.

If you wait for the jump ship to fly through the air for 2,000 feet and it has zero ground speed, you will have zero separation, not 2,000 feet.

You will not have zero seperation at opening time! I am assuming the upper winds at at ~80kts, the same speed as the aircraft

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That would work great in court.... Sorry judge I was not speeding, you see I was in reverse gear which means I was doing -40mph.

BTW If you do a jump run with the wind do you then have a negative ground speed?

If a Piper Cub is flying with a 40 knot airspeed into a 45 knot head wind, it has a –5 knot ground speed. If a jump ship is moving with the wind and not into the wind, relative to the ground, it has a negative ground speed.

What would you call it if a jump ship was backing up over the ground on jump run? “Going forward in reverse”? Sounds like something the French Army would say.

Derek

As I said, it's all been done before and although the resources are out there, some people just don't seem to use them.

I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed.

I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed.

This was all figured out many, many years ago....

So if the jump ship has zero ground speed (130 fps-130fps), then it might a well be a 13,500 tower. If someone jumps off a 13,500 foot tower and then someone else jumps off the same tower 15 second later, they will open at the same exact point, as they will drift the exact same amount. The only separation will be from the first’s person’s canopy speed as they fly for 15 seconds.

And there would be no forward throw, since the jumper has no forward momentum.

Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP....

The only separation will be from the first’s person’s canopy speed as they fly for 15 seconds.

That's right! With the wind blowing at 130 ft/sec that's some distance they will cover before the next jumper arrives and opens. The 15 second delay allows the 1st jumper to be blown horizonataly away before the next jumper exits. They both follow the same trajectory but are still in their own vertical column of air.

Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP....

Um, no, it will drift down wind, just like the jumpers exiting from the 13,500 foot tower would. If you dropped 2 balls from the tower, 15 seconds apart they would hit the same spot on the ground. I.e. no seperation.

That's right! With the wind blowing at 130 ft/sec that's some distance they will cover before the next jumper arrives and opens. The 15 second delay allows the 1st jumper to be blown horizonataly away before the next jumper exits. They both follow the same trajectory but are still in their own vertical column of air.

That means they will open in the exact same airspace with only the 15 seconds of canopy speed for separation. If a group of 4 exits first, then another group of 4, then the second group of four will be opening within the first group’s airspace. That is zero separation. We do not rely on a canopy’s airspeed for separation. What if the first jumper has line twists and flies into the wind and the second jumper has line twists and flies downwind? They will have a high chance of eating up that 15 seconds and having a collision.

Separation means opening points are separated not the same opening point with canopy speed used for separation. The same opening point means there is zero separation.

If you dropped 2 balls from the tower, 15 seconds apart they would hit the same spot on the ground. I.e. no seperation.

Thankfully we are parachutists and we don't hit the ground That is why ground speed is irrelevant. We need to look at what the plane/tower is doing relevant to point (air mass) where we open our canopies.

If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them.

Try to think about the canopy speed as the speed of the canopy due to wind drift and not it's own speed.

After I got put down by Ron & Kallend I invested a lot of time reading the stuff Kallend put on the web. I believe at least for some people (me included) there are some ways in which we think that are fundamentally wrong. I finally figured out what was going on after I stopped thinking about the ground. The ground is irrelevant!

If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them.

Distance = Velocity * Time

Distance = 130 ft/sec * 15 sec = 1950 ft

If jumper one opens with line twists and flies upwind (the canopy’s airspeed minus the 130 fps wind speed gives jumper one’s ground speed) at time t and jumper 2 opens in the exact same airspace (which is the definition of no separation) at t+15 with line twists and flies down wind (for a ground speed of 130 fps plus the canopy’s airspeed). They have very little separation and may have a collision since they are flying towards each other at their combined airspeeds.

Canopies should have opening points some distance apart.

If canopy speed was sufficient for separation, why track at break off? Why not just turn away and pull and let the canopy’s airspeed provide the separation.

Just to clarify, the ground is irrelevant only until after opening. Exit separation must be increased in higher winds due to the increased speed and reduced time it takes under canopy to get to the target, i.e. due to congestion (assuming an upwind opening point).

When I think about exit seperation on a jump run I am not normally thinking about the problems of congestion when landing the canopy. I am mainly concerned that each jumper has his own column of air to use.

The distance between the two canopies would be the same no matter what the groundspeed is, assuming a constant airspeed. The closing time for the two canopies would be the same as well (yes, regardless of groundspeed).

I'm not jumping any canopy in 88.6 mph (130 fps, 77.07 knot) winds. I used that number because it's a reasonable airspeed for a plane on jumprun. For the 15 seconds you wait on the plane, you are moving away from the person who got out before you at approximately the airspeed of the plane. Not some arbitrary groundspeed.

Scenario 2 has a 10-knot increase in airspeed over scenario 1. By your reasoning, less time needs to be left between groups in scenario 2 because of the 10 knot increase in airspeed. Both scenarios have a 10 knot ground speed.